1. Technical Field
A certain aspect of embodiments discussed herein is related to a cryogenic refrigerator that includes a displacer that has a groove formed on its periphery.
2. Description of the Related Art
In general, refrigerators including a displacer, such as Gifford-McMahon (GM) cycle refrigerators and Stirling cycle refrigerators, are known as cryogenic refrigerators that produce cryogenic temperatures at or below 15 K.
Taking the GM refrigerator as an example, the displacer is so provided in a cylinder as to be able to reciprocate in the cylinder, and an expansion space and a room temperature space are provided at a low temperature end and a high temperature end, respectively, inside the cylinder. Further, a gas passage through which a refrigerant gas (helium gas) flows is provided inside the displacer. This gas passage is filled with a regenerator material, and communicates with the expansion space and the room temperature space.
At the gas supply process, a refrigerant gas is supplied from a compressor to the room temperature space at the high temperature end, and this high-pressure refrigerant gas is introduced into the expansion space through the gas passage inside the displacer. At the gas return process, the refrigerant gas inside the expansion space is returned to the compressor through the same passage.
In this configuration, cold temperatures are produced in the expansion space by optimizing the timing between the reciprocation of the displacer and the supply and return process of the refrigerant gas. The refrigerant gas cooled by the produced cold temperatures cools the regenerator material inside the displacer when the refrigerant gas is returned to the compressor through the displacer at the gas return process. Further, at the gas supply process, the refrigerant gas is introduced into the expansion space after being cooled by the regenerator material.
A gap is formed between the displacer and the cylinder to allow the displacer to reciprocate inside the cylinder. However, if the refrigerant gas passes through this gap to flow directly between the room temperature space and the expansion space, the cooling efficiency is reduced because of the absence of cooling by the regenerator material. As an example, this may be prevented by providing a sealing mechanism that prevents a flow of the refrigerant gas in the gap between the cylinder and the displacer. In general, an O-ring is used as this sealing mechanism.
However, this type of sealing mechanism may degrade over time to reduce its sealability. In this case, with this type of sealing mechanism, a desired refrigeration capacity cannot be achieved. Therefore, it has been proposed to form a helical groove on the outer peripheral (circumferential) surface of the displacer instead of providing a sealing mechanism such as an O-ring. (See, for example, Japanese Patent No. 2659684.)